Compartmental analysis of calcium metabolism in very-low-birth-weight infants

Use of stable isotopes and mathematical modelling to investigate human mineral metabolism

Individuals have varying needs for minerals that are dependent, amongst other things, on their lifestyle, age and genetic makeup. Knowledge of exact individual nutritional requirements should lead to better health, increased quality of life and reduced need for expensive medical care. Bioavailability, nutrient-gene interactions and whole-body metabolism all need to be investigated further if we are to progress towards the goal of defining optimal health and nutritional status. The discussion which follows will critically review the latest developments in the area of metabolism for several of the minerals that are essential for human health: Ca, Zn, Cu and Se. Stable-isotope tracers and mathematical modelling are some of the tools being used to facilitate the greater understanding in uptake, utilisation and excretion of these minerals. Stable isotopes, administered in physiological doses, present little or no risk to volunteers and allow metabolic studies to be carried out in vulnerable population groups such as children and pregnant women. Intrinsic labelling of foodstuffs ensures that the tracer and the native mineral will behave similarly once inside the body. Advances in computing power and software dedicated to solving nutritional problems have made it possible for investigators to use mathematical modelling in their experimental work. Mineral metabolism is ideally suited to a form of modelling known as compartmental analysis, which allows rates of mineral transferand sizes of mineral stores to be calculated accurately without the need for invasive sampling of body tissues.

Total calcium absorption is similar from infant formulas with and without prebiotics and exceeds that in human milk-fed infants

Background

1) To evaluate calcium absorption in infants fed a formula containing prebiotics (PF) and one without prebiotics (CF). 2) To compare calcium absorption from these formulas with a group of human milk-fed (HM) infants.

Methods

A dual tracer stable isotope method was used to assess calcium absorption in infants exclusively fed CF (n = 30), PF (n = 25) or HM (n = 19). Analysis of variance was used to analyze calcium intake, fractional calcium absorption, and the amount of calcium absorbed.

Results

Calcium intake (Mean ± SEM) for PF was 534 ± 17 mg/d and 557 ± 16 mg/d for CF (p = 0.33). Fractional calcium absorption was 56.8 ± 2.6 % for PF and 59.2 ± 2.3 % for CF (p = 0.49). Total calcium absorbed for PF was 300 ± 14 mg/d and 328 ± 13 mg/d for CF (p = 0.16). For HM infants calcium intake was 246 ± 20 mg/d, fractional calcium absorption was 76.0 ± 2.9 % and total calcium absorbed was 187 ± 16 mg/d (p <0.001, compared to either PF or CF).

Conclusions

Despite lower fractional calcium absorption of CF and PF compared to HM, higher calcium content in both led to higher total calcium absorption compared to HM infants. No significant effect of prebiotics was observed on calcium absorption or other markers of bone mineral metabolism.

Calcium absorption in very low birth weight infants with and without bronchopulmonary dysplasia

OBJECTIVE

To evaluate the effects of early bronchopulmonary dysplasia (BPD) on calcium (Ca) metabolism and growth in very low birth weight (VLBW) infants.

STUDY DESIGN

A dual-tracer, stable isotope method was used to assess Ca absorption in VLBW infants. Infants with early BPD received energy-dense feedings and mild fluid restriction.

RESULTS

Sixteen of 41 preterm infants were classified as having early BPD. Fractional Ca absorption (early BPD, 58.4 ± 4.6% versus no early BPD, 50.3 ± 4.0%, P = .2), total Ca absorption (early BPD, 127 ± 14 mg/kg/d versus no early BPD, 104 ± 9 mg/kg/d, P = .9), and Ca retention (early BPD, 99.6 ± 10.0 mg/kg/d versus no early BPD, 91.0 ± 9.8 mg/kg/d, P = .2) were similar among groups. There was no significant difference in weight gain, linear growth, or head circumference growth between groups.

CONCLUSIONS

The ability of VLBW infants with early BPD and fluid restriction to grow and accrete calcium is similar to those without early BPD. The use of high caloric density feedings in VLBW infants with early BPD can help achieve bone and overall growth outcomes close to those achievable in utero.

Calcium nutrition and metabolism during infancy

Calcium is a vital mineral for the developing newborn infant. This review discusses perinatal and neonatal calcium metabolism, with an emphasis on enteral calcium absorption and the nutritional factors affecting calcium bioavailability including the three major endocrine hormones involved in calcium metabolism: parathyroid hormone, vitamin D, and calcitonin. The placenta transports calcium to the fetus throughout pregnancy, with the largest amount of fetal calcium accumulation occurring in the third trimester. At birth, the newborn transitions to intestinal absorption to meet the body's calcium needs. Most calcium is absorbed by paracellular passive diffusion in the small intestine. Calcium intestinal absorption is affected by the type and amount of calcium ingested. It is also affected by the amount of intestinal calcium that is bound to dietary fats and proteins. One major consequence of decreased calcium absorption is metabolic bone disease in which there is a failure of complete mineralization of the bone osteoid.

Urinary excretion of calcium and phosphate in preterm infants

The aims of this study were to determine reference ranges for the urinary calcium (UCa/Cr) and phosphate (UPO(4)/Cr) creatinine ratios and to study factors influencing these ratios in a representative population of preterm infants managed according to current nutritional guidelines. Spot urine samples were obtained from 186 preterm infants (gestation 24-34 weeks) for measurement of UCa/Cr and UPO(4)/Cr ratios as part of a routine metabolic bone screening program, once every 2-4 weeks from the 3rd to the 18th week of life. Data were also collected on gender, appropriate or small for gestational age (SGA), nutrition [total parenteral nutrition (TPN), preterm or term formula, and breast milk], plasma Ca, P0(4), urea, and electrolytes and on the use of drugs (frusemide, dexamethasone, and theophylline). Data from infants treated with any of these three drugs were analyzed separately and not included in establishing the reference ranges for UCa/Cr and UPO(4)/Cr. The mean gestational age of the study population was 28 weeks (range 24-34 weeks). The 95th percentile for UCa/Cr at 3 weeks of age was 3.8 mmol/mmol and decreased significantly with increasing postnatal age (P<0.001). The 95th per-centile for UPO(4)/Cr was 26.69 mmol/mmol at 3 weeks of age, but this did not change significantly with increasing postnatal age (P=0.296). On univariate analysis there was no significant association of UCa/Cr and UPO(4)/Cr with gender and type of enteral nutrition. The UCa/Cr was lower in infants who were SGA (P=0.013) and with low plasma Ca (P=0.008). Infants on TPN had significantly higher UCa/Cr (P =0.019) and lower UPO(4)/Cr ratios(P<0.001). Multivariate analysis confirmed the decrease in UCa/Cr ratio with increasing postnatal age, but the SGA effect was eliminated. The use of furosemide(P<0.001) and theophylline (P=0.003) was associated with a significant increase in the UCa/Cr ratio. The use of dexamethasone was also associated with an increase in UCa/Cr ratio, but this did not achieve statistical significance (P=0.339). The use of furosemide, theophylline,and dexamethasone had no effect on UPO(4)/Cr. We report a reference range for UCa/Cr and UPO(4)/Cr ratios and factors influencing these ratios in a representative population of preterm infants between 24 and 34 weeks gestation, managed according to current nutritional guide-lines.

Calcium kinetics are altered in clinically stable girls with cystic fibrosis

Reduced bone mass in individuals with cystic fibrosis (CF) may result from alterations in calcium metabolism. Bone calcium deposition and resorption rates, calcium balance, and markers of bone turnover were assessed using stable isotopes of calcium in 22 prepubertal and pubertal girls with CF. Bone calcium deposition was associated with the availability of dietary calcium, total serum osteocalcin, and leptin concentrations. Reduced bone mass in individuals with CF may result from inadequate bone calcium (Ca) deposition, and excessive resorption, although these parameters have not been directly assessed in children with CF. We used stable Ca isotopes to measure rates of bone Ca deposition (Vo+), resorption, and retention in 22 clinically stable girls with CF (aged 7-18 yr). Rates of bone Ca deposition were determined by mathematically modeling the disappearance of iv Ca stable isotope ((42)Ca) for 6 d post dosing. Indirect markers of bone turnover and hormones associated with pubertal development were also assessed. Rates of bone Ca deposition and retention were highest during early puberty (Tanner stages 2 and 3). Calcium deposition rates in prepubertal (Tanner 1) and postmenarchal girls (Tanner stages 4 and 5) did not support substantial bone Ca retention. Net absorption of dietary Ca and serum osteocalcin and leptin concentrations were positively associated with Vo+. Time post menarche and serum leptin concentrations explained 91% of the variability in Vo+ (P = 0.0007). Serum total osteocalcin was low (10.9 +/- 5.4 ng/ml), and a substantial percentage of osteocalcin was undercarboxylated (54.3 +/- 11.8%). We concluded that increased calcium absorption and serum leptin concentrations were significantly associated with rates of bone Ca deposition, demonstrating an impact of nutritional status on this process. Rates of bone Ca deposition were lower than typically reported in healthy children, as were indirect markers of bone formation. These alterations in bone turnover contribute to reduced bone mass in girls with CF.

Using stable isotopes to assess mineral absorption and utilization by children

Adequate mineral intake is a crucial part of a healthy diet for children-it supports appropriate growth and development and provides protection against childhood conditions like anemia and helps to prevent future adult diseases such as osteoporosis. Challenges in performing and interpreting studies in infants and children have hampered the accurate assessment of their mineral utilization. Many of the most powerful techniques used in adults, such as radioisotope testing, are not appropriate for use in children. In recent years, advanced mineral stable-isotope techniques have been developed to fill this gap. Pediatric applications include studies of calcium absorption and kinetics during puberty and evaluation of the calcium-iron interaction in infants and toddlers. The effects of genetics in determining calcium absorption and bone turnover may become an important research area. The goals and methods of ongoing mineral stable-isotope research in infants and children are examined in this report. In the past, the cost and difficulties in obtaining isotopes have limited such research. This situation has improved considerably, although relatively few nutrition research laboratories are prepared to perform sample analyses.

Effect of growth hormone treatment on calcium kinetics in patients with osteogenesis imperfecta type III and IV

Using a dual stable isotope technique, the effect of growth hormone (GH) on whole body calcium (Ca) metabolism was studied in children (ages 5-14 years) with type III (n = 9) and IV (n = 8) osteogenesis imperfecta. Each subject was studied twice: at baseline and following a GH (0.1-0.2 U/kg per day) treatment period of 1-1.5 years. Subjects were given 42Ca intravenously and 44Ca orally. The sera and urine 42Ca and 44Ca isotopic enrichments were followed over 7 days using thermal ionization mass spectrometry. The SAAM program was used to fit a three-compartment model to the tracer data. No significant differences were observed between: (1) children with type III and IV disease; or (2) baseline studies of boys and girls within each disease type. However, GH treatment significantly increased: (1) the exchangeable calcium pool (EP) in type III patients (2086 vs. 4422 mg/day, p = 0.02); and (2) the parameter associated with bone calcium accretion in type IV patients (Vo+: 973 vs. 1560 mg/day,p = 0.03) with boys responding with a significantly greater increase than girls (p = 0.008). Although not statistically significant, a trend toward an increase in Vo+ in type III patients and in EP in type IV was observed following treatment. Our observations imply that more Ca was available for bone mineralization following GH treatment in these subjects.

Nutritional aspects of calcium absorption

The amount of calcium absorbed in the intestine depends on habitual calcium intake. When intake is low, active transcellular calcium transport in the duodenum is upregulated and a larger proportion of calcium is absorbed by the active process than by the passive paracellular process that prevails in the jejunum and ileum. Bioavailability of the calcium source-digestibility and solubilization-plays a role under conditions of low calcium intake but is relatively unimportant when calcium intakes are high (e.g. >800 mg/d in people). Vitamin D intake is a second factor, as active calcium transport is directly and proportionally dependent on the presence in the intestinal cell of calbindin D9k, the biosynthesis of which is totally vitamin D dependent. Passive absorption in jejunum and ileum is the major absorptive process when calcium intake is adequate or high. Passive calcium absorption is a complicated function of solubility in the distal small intestine, the length of sojourn of the chyme in a given intestinal segment, and the rate of paracellular diffusion from lumen to lymph and blood. Calcium that reaches the large intestine undergoes absorption there by both active and passive processes. Probably no more than 10% of total calcium absorption takes place in the large intestine, whether calcium intake is low or high. Calcium absorption by the large bowel can assume nutritional importance under conditions of significant small bowel resection.

Bone mineralization status measured by dual energy radiographic densitometry in preterm infants fed commercial formulas

We have studied the effect of two preterm commercial infant formulas with different calcium and phosphorus contents on the mineral balance and bone mineralization of 30 preterm infants at 1 month of age. Bone mineralization was measured by dual energy X-ray densitometry. The formula supplying a higher content of calcium and phosphorus promoted higher mineral retention (P<0.01) as well as higher bone mineral content (1.556 vs. 1.073 g, P<0.01) and bone mineral density (0.458 vs. 0.424 g/cm2, P<0.05), approaching values of the control group, which comprised a cohort of 15 preterm newborns whose gestational age was 4 weeks older than the subjects selected to be fed with the formulas. The intake of calcium correlated with retention (r=0.69); the phosphorus intake also correlated with phosphorus retention (r=0.95). Intakes of calcium and phosphorus correlated with the bone mineral content (r=0.65) and with bone mineral density (r=0.49). We conclude that formulas for preterm infants should not have a calcium content lower than 120 mg/100 kcal and should have a calcium/phosphorus ratio of about 2 to promote adequate bone mineralization.

Insights into bone metabolism from calcium kinetic studies in children

Changes in the mineralization rate of the skeleton during childhood are related to normal growth and pubertal development. These may be affected by genetic factors, including race and gender, and by the presence of abnormalities of growth or hormonal abnormalities such as occur in children with chronic illnesses. We have used multicompartmental studies to examine calcium kinetics in healthy children ranging in age from premature infants of 1-2 kg body weight through adolescence. These studies are performed using orally and intravenously administered stable isotopes of calcium. Sample collection requires multiple blood samples to be obtained during the initial time period after isotope dosing, which is feasible in older children using an indwelling catheter. We have found that the peak rate for both bone calcium deposition and removal occurs in girls during the year before menarche. Peak kinetic rates decrease in an exponential fashion post-menarche. On a body-weight basis, the greatest rates of bone calcium deposition and removal are in infants, especially premature infants.

Development and regulation of calcium metabolism in healthy girls

The major components of calcium metabolism, as evaluated by a dual-tracer stable isotope method, were determined in 100 studies of 68 healthy girls, aged 5-18 y and analyzed from a developmental and regulatory viewpoint. Bone calcium deposition and removal rates were closely correlated with the size of the exchangeable bone calcium compartment. All three quantities, as well as intestinal calcium absorption, peaked at or near menarche. Both bone calcium deposition and removal rates were positively and linearly correlated with calcium absorption. However, in this correlation, because bone calcium deposition increased 70% faster than calcium absorption, most of the increase in the bone calcium compartment and its turnover must have occurred in response to something other than intestinal calcium input; presumably this occurred in response to developmental signals. Nevertheless, the constancy of the serum calcium in the face of a large intestinal calcium input and the modest way in which excretion overcame the calcium load in this population point to the importance of the exchangeable bone calcium compartment, in dynamic equilibrium with the bone mineral, as the site at which most of the load is taken up. In this population of girls, as in older women, this increase in the skeletal calcium balance resulted from a decrease in the bone calcium removal rate that was greater than the corresponding increase in the bone calcium deposition rate.

Randomised controlled trial of a synthetic triglyceride milk formula for preterm infants

AIMS

To test whether use of infant formula containing synthetic structured triglycerides results in: (i) increased palmitate absorption; (ii) increased total fat absorption; (iii) reduction in calcium soap formation in the gut; and hence (iv) increased calcium absorption.

METHODS

A randomised study was made of 24 infants comparing three formulas, one containing the synthetic fat Betapol with 74% of palmitate in the 2-position, which was substantially higher than in the two comparison diets (8.4% and 28%). The hypothesised outcomes were tested using balance studies, detailed chemical analysis of stool specimens and dual calcium isotope tracers (44calcium orally and 46calcium intravenously).

RESULTS

Three of the four hypotheses were confirmed: use of a formula rich in 2-position palmitate (i) improved palmitate (16:0) and also (18:0) absorption; (ii) reduced the formation of insoluble calcium soaps in the stool; and (iii) improved calcium absorption, determined by the dual tracer technique from 42 (SE 3)% to 57 (7)%.

CONCLUSION

Synthetic triglycerides that mimic the stereoisometric structure of those in breast milk may have a valuable role in the design of formulas used for preterm infants in neonatal intensive care units.

Absorption of calcium, zinc, and iron from breast milk by five- to seven-month-old infants

Data are scarce regarding mineral bioavailability from human milk in older infants who may also be receiving solid foods (beikost). We measured the absorption of Ca, Zn, and Fe in 14 healthy, nonanemic 5-7-mo-old breast-fed infants whose mothers milk was extrinsically labeled with stable isotopes (44Ca, 70Zn, and 58Fe) of these minerals. In addition, Ca and Zn stable isotopes (46Ca and 67Zn) were administered i.v., and a second isotope of Fe (57Fe) was given orally without food as a non-meal dose. Subjects were not receiving any artificial infant formula or cow's milk, but most (10/14) were receiving beikost. Ca and Zn absorption was calculated using the urinary excretion of the isotopes during the 24 h after dosing (Ca) or their urinary ratio 72 h after dosing (Zn). Fe absorption was calculated using the red blood cell incorporation at 14 d. Fe absorption averaged 20.7 +/- 14.8% from the 58Fe given with human milk (geometric mean, 14.8%) and 17.7 +/- 15.1% (geometric mean, 11.0%) from the 57Fe non-meal dose. Ca absorption averaged 61.3 +/- 22.7% and Zn absorption (n = 10) averaged 49.5 +/- 18.5%. Absorption of Fe (natural logarithm) from the non-meal Fe dose (57Fe) but not from the human milk (58Fe) was significantly negatively correlated to serum ferritin (r = -0.70, p = 0.007 versus r = -0.35, p = 0.24). At the intake levels in this study, total daily Fe, Ca, and Zn intakes from beikost were not significantly correlated to their fractional absorption from breast milk, but Fe intake from beikost was significantly negatively correlated to absorption of Fe from the non-meal dose (r = -0.61, p = 0.021). We conclude that minerals are well absorbed from human milk in older infants after the introduction of beikost to the diet.